1. Field of Invention
The present invention relates to a MOS solid-state imaging element of a threshold voltage modulation system.
2. Description of Related Art
A variety of electronic equipment, such as cellular phones and digital cameras provided with a miniaturized camera have become increasingly popular. As an example of an imaging element for such a camera, a MOS solid-state imaging element that is referred to as an imaging element of a threshold voltage modulation system has been developed. See, for example, Japanese Patent Publication Laid-Open No. 11-195778.
When an imaging element of a threshold voltage modulation system is compared to a CCD (Charge Coupled Device) solid-state imaging element that has the same pixel size and pixel number as it, the power consumption thereof is lower than that of a CCD solid-state imaging element although it is inferior to a CCD solid-state imaging element in optical sensitivity. In addition, compared to a CMOS (Complementary Metal Oxide Semiconductor) solid-state imaging element that has the same pixel size and pixel number, an imaging element of a threshold voltage modulation system has higher optical sensitivity although the power consumption is the same.
An imaging element of a threshold voltage modulation system has a special pixel structure in order to enhance the optical sensitivity thereof more than that of a CMOS solid-state imaging element and improve the image quality. Specifically, in one pixel (it is also referred to as “unit pixel” hereinafter) of a CMOS solid-state imaging element, three (3) or four (4) transistors are used in order to convert charges that were photoelectrically converted in response to the amount of received light by a photo diode (they are also referred to as “photocharges” hereinafter) into voltage (it is referred to as “charge-voltage conversion” hereinafter) and amplify it. Meanwhile, in one unit pixel of an imaging element of a threshold voltage modulation system, only one transistor is used. Furthermore, a structure that is special and is different from that of a general MOS transistor is used for this one transistor. Specifically, a hollow of energy that is referred to as a carrier pocket (it is also referred to as “well”) is formed in the transistor. This carrier pocket has a structure where potential against, for example, a hole is lowered. Therefore, holes (they are also referred to as “photoholes”) of photocharges generated in a photo diode is accumulated. The threshold of a transistor changes depending on the number of photoholes accumulated in a carrier pocket such that the source potential of a source follower formed of this transistor changes. Therefore, pixel data depending on the amount of received light can be detected by reading out source potential that changes depending on the number of photoholes accumulated in a carrier pocket.